The subject matter of this invention relates generally to voltage regulator circuits and relates more specifically to the static VAR generator flicker control type voltage regulator. Static VAR generators, whether used as voltage regulators or flicker compensators, compensate for the effects of voltage change on the terminals of an AC electrical system. The flicker compensator typically is utilized where the load supplied by the AC system varies widely from half cycle to half cycle in a generally unpredictable manner. Two well-known types of loads which cause this effect are electric arc furnaces and electric saw mill motors. The word "flicker" derives from the effect that the widely varying loads have on the light bulbs of unregulated or uncompensated circuits which are connected to the same power supply line as the varying load. The static VAR generator or flicker compensator includes a control circuit which measures the magnitude of arc furnace currents in consecutive half cycles of the line or supply voltage. The measured information is used to compute necessary compensating currents in terms of required firing angle signals for the thyristors of the static VAR generators. The static VAR generator output therefore, can be adjusted once during each half cycle per phase. Generally, a great area of interest in the art of static VAR generators is concerned with apparatus and method for determining the proper firing angle for the inductor control thyristors of the VAR generator. Examples of this can be found in U.S. Pat. No. 3,936,727, issued Feb. 3, 1976 to F. W. Kelly, Jr., and G. R. Lezan, which teaches a compensation control device which determines firing time for a static switch in accordance with the magnitude of the reactive load current and which additionally teaches a regulating means which controls the firing time to maintain the line current and line voltage at a selected line location in substantial phase coincidence. Furthermore, U.S. Pat. No. 3,999,117, issued Dec. 21, 1976 to Gyugyi et al, teaches a static VAR generator and compensator where time delayed firing angles are calculated from integrating furnace load currents over predescribed intervals during real time to thus maintain balance load current at specified phase angles. Other patents in this area include U.S. Pat. No. 4,000,455 issued Dec. 28, 1976 to Gyugyi et al; U.S. Pat. No. 4,068,159, by Gyugyi et al; and U.S. Pat. No. 4,172,234 also by Gyugyi et al. In the past, most of the emphasis has been placed on utilizing load current to determine the time at which compensating reactive power is applied to a system to be regulated or compensated. These circuits utilize a thyristor controlled inductor. The operation of the thyristor controlled inductor is well known. As the firing or pulse delay angle (.alpha.) is increased, the current in the inductor decreases. The total current drawn from the network is the difference between a fixed capacitor current, where the capacitor is connected in parallel with the combination of the thyristor and the inductor, and the variable inductor current. This effect can produce leading or lagging reactive power depending on the relative size of the inductor and capacitor. It would be advantageous if an AC voltage regulating loop which is related to the magnitudes of the line to line voltages could be provided for utilization in calculating thyristor control angles .alpha.. Furthermore, it would be advantageous if the closed loop voltage control system could utilize apparatus therein which was essentially fast and utilized linear-acting rather than second or third order control elements. It would also be advantageous if the compensator or regulator could effectively control flicker without necessarily effecting relatively slow changes in line voltage which are generally tolerable. The conventional method of obtaining the error of an AC voltage feedback signal is by full wave rectification of the line voltage and the consequent subtraction of the rectified signal from a DC reference signal. The resulting error signal, .DELTA.V, consists of an average value and a superimposed rectifier ripple which contains harmonics in a well-known composition. The useful error is represented by the average signal only. This must be extracted from the overall signal. The ripple tends to be considered as noise. Unfortunately, the average value which is contained in the error signal is typically two orders of magnitudes smaller than the inherent ripple amplitude. Therefore, in the conventional apparatus, a low pass filter is usually employed to remove all undesirable harmonics from the overall signal. Due to the heavy filtering needed, the error signal generation and dynamic compensation of the servo loop results in a trade-off between a relatively slow response time with sufficient ripple filtering and decreased ripple filtering but faster response time.
Even though the trade-off capability exists with the utilization of the low pass filter, the ultimate design choice usually leads to the elimination of the 120 Hz first harmonic produced by the rectifier. Incidently, higher order subharmonics are additionally removed and the most significant effects of transient or spurious signals are removed necessarily because of the low pass filter action. One of the things desired in any kind of closed loop feedback system is relatively fast response time. It has been found that the VAR generator power delivering circuit, i.e. thyristors, capacitors, and inductors, has an inherent transport lag which is generally difficult to improve. The calculating time, however, is a function of the switching or control circuit, and that is improvable. It would be desirous, therefore, if the control portion or switching portion of the VAR generator feedback system could be speeded up. In so doing, it has been determined that the low pass filter is a prime candidate for removal. However, if the low pass filter is removed, the 120 Hz harmonic as well as all subharmonics and transient and noise signals will not be removed. This has a tendency to cause an erroneous feedback signal which will lead to under or over compensation with regard to the desirable range of voltage regulation and flicker control. It will be desirable, therefore, to find some way of removing the harmonics of the 60 Hz signal without the utilization of low pass filters. Finally, it has been found that a fixed voltage reference signal is usually necessary as part of the voltage feedback control. However, if a fixed voltage reference is utilized, it will cause the power circuit portion of the reactor VAR generator to provide compensating power for relatively slow voltage variations. The relatively slow voltage variations as mentioned previously are often not considered a serious problem. However, if compensation is provided by the VAR generator to remove them, a significant increase in the size or capacity of the VAR generator is required, that is, the VAR capacity must be greater than if flicker control only is desired. It would be advantageous therefore, if an adaptive voltage reference signal could be utilized which essentially allowed the switching portion of the feedback network to ignore relatively slow changes in voltage but to supply appropriate feedback control where the frequency of change is in the range in which it is desired to control flicker or provide some other form of voltage regulation. It would be further advantageous if the utilization of the adaptive voltage reference signal could be implemented in such a way as to eliminate the spiking problem often associated with certain types of voltage feedback control systems.